In HVDC systems in general, and in particular in multi-station systems, there is a need of a d.c. breaker for example for isolating a faulty unit (converter, d.c. filter, d.c. pole, etc.) without having to interrupt the operation of the pole in question in all stations concerned. A plurality of d.c. breaker devices have been proposed, but all of them exhibit disadvantages, and none of the proposals has been used to any significant extent. A special problem with HVDC plants is the presence of smoothing reactors, which with their high inductance, typically 300-400 mH, render the interruption of a direct current in the plant very difficult.
Thus, from ETZ-A (89) 1968, No. 18, pp. 421-423, a d.c. breaker is known in which a circuit breaker is used to commutate the current from the main current path to an energy-absorbing parallel resistor. The residual current through the resistor is interrupted in this known breaker by means of a series-connected d.c. breaker of a special design. This residual current breaker must be designed to break at full line voltage. Such special d.c. breakers are expensive designs and require high development costs.
From U.S. Pat. No. 3,809,959 a d.c. breaker device is previously known, which has two series-connected mechanical breaks, for example breaks in an a.c. circuit breaker. A first break is connected in parallel with a varistor and with a capacitor in series with a spark gap. For breaking the current, this break is opened, whereby the current is intended to be transferred to the capacitor. This causes the voltage of the capacitor to grow rapidly and reach the knee voltage of the varistor, whereupon the varistor is to take over the direct current. The varistor voltage constitutes a counter voltage which drives the direct current in the circuit towards zero, whereupon the second break may be opened to obtain a galvanic insulation. An HVDC breaker of this prior art type has the disadvantage that the current through the break has no natural zero crossing, and the arc in the breaking member is in most cases either stable or insufficiently stable. This means that difficulties arise in obtaining a problem-free transfer of the current from the break to the capacitor, that is, difficulties in obtaining a satisfactory breaking function.
In the CIGRE report 14-201, 1990, p. 7, FIG. 10 with associated text, an HVDC breaker of the type stated in the introductory part of this description is proposed, in which the first break mentioned in the preceding paragraph is replaced by a gate turn-off thyristor connection, for example a series connection of gate turn-off thyristors (GTO thyristors). The thyristor connection must be dimensioned to take up all of the recovery voltage after the turn-off. It must, therefore, in practice consist of a large number of series-connected GTO thyristors. The thyristor connection continuously carries the current flowing through the d.c. breaker during normal operation, and the continuous current in practice also necessitates a parallel connection of GTO thyristors. The number of thyristors in the connection will thus be high. The thyristor connection and hence the d.c. breaker device therefore become complicated and expensive. Further, because of the continuous current, the losses and hence the costs due to the losses are high in the thyristor connection. The high continuous losses also necessitate an amply dimensioned cooling system for the thyristors. Finally, means are required for transfer of firing and turn-off signals between ground potential and the potential where the thyristor connection is arranged. All of these factors result in a d.c. breaker device of this proposed type becoming expensive and complicated.
U.S. Pat. No. 3,777,179 describes a d.c. breaker with first and second mutually series-connected mechanical breaks which are connected in parallel with a capacitor and which normally carry the load current of the breaker. The first break is connected in parallel with a gas discharge device which can be electromagnetically controlled to non-conducting state. Upon opening the breaks, the discharge device takes over the current through the first break. After deionization thereof, the discharge device is turned off, the capacitor takes over the load current and is charged to a counter voltage, and the second break is deionized. Gas discharge devices of the kind stated have proved to be less suitable for practical operation. Further, this device has the disadvantage that the rate of change of the current is high in connection with the zero crossing of the current through the second break. This results in the time for deionization of this break becoming short and in the voltage across the break growing rapidly. These factors cause the maximum voltage and current level, at which a certain break can be used, to become limited.
U.S. Pat. No. 3,611,031 describes a d.c. breaker of, in principle, the same configuration and function as the breaker described in the preceding paragraph.
Derwent Abstract No. 83-734248/32, week 8332. Abstract of SU-964-758-A, describes a device of a similar construction, in which the first break is connected in parallel with a gate turn-off thyristor connection which takes over the load current upon opening of the breaks, for example at a short circuit. When turning off the thyristor, the current thereof is taken over by a resistor which is connected in parallel with the first break and which limits the short-circuit current. However, no zero crossing of the current through the second break is obtained, and the device is therefore not suitable for use as a d.c. breaker. Further, the gate turn-off thyristor connection must be dimensioned for high voltage and becomes bulky and expensive.
U.S. Pat. No. 4,216,513 describes a d.c. breaker in which a mechanical break is connected in parallel with a series connection of a capacitor and an inductor. Under certain conditions regarding the load current and regarding circuit data, upon opening of the break, because of the negative current-voltage characteristic of the arc occurring, a natural oscillation with an increasing amplitude will be generated in the oscillating circuit formed by the capacitor, the inductance and the break. When the amplitude becomes equal to the load current, the resulting current through the break becomes zero, and a deionization can take place. Because of the dependence of the breaking operation on the prevailing current and on the prevailing circuit data, this device gives no reliable and controlled breaking under practical operating conditions.